1. Technical Field of the Invention
The present invention relates to a fixing apparatus, an image forming apparatus and a fixing method, and particularly to a fixing apparatus for fixing a toner image transferred on a recording sheet or the like, an image forming apparatus and a fixing method.
2. Related Art
In an image forming apparatus, such as a copying machine or a printer, adopting an electrophotographic system, a fixing apparatus for fixing a toner image transferred on a recording sheet is used. As one of modes of the fixing apparatus, there is a heat-pressure fixing apparatus of a fixing belt system.
In this fixing apparatus, an endless fixing belt is wound around a heating roller and a fixing roller, and a pressing roller adjacent to the fixing roller and the fixing roller are brought into press contact with each other through the fixing belt.
A heater is disposed inside the heating roller, and the fixing belt is heated when it moves on the heating roller. The heated fixing belt reaches a nip part (gap between two rollers) formed of the fixing roller and the pressing roller and comes in contact with a recording sheet passing through the nip part, so that an unfixed toner image on the recording sheet is fixed by the heat and pressure.
In order to uniformly perform the fixation of the toner image to the recording sheet, it is important to uniformly keep the temperature of the heating roller within a suitable range. Thus, various techniques relating to heater control inside the heating roller have been conventionally proposed.
For example, JP-A-7-244439 discloses a technique in which plural heaters are provided in a heating roller and the on and off of each of the heaters is independently controlled.
Besides, JP-A-10-301439 discloses a technique in which the periods and ratios of the on and off of plural heaters are selected from a specified range according to the paper quality or temperature and humidity, and the user can suitably change them.
Besides, JP-A-8-234617 and U.S. Pat. No. 5,872,620 discloses a technique in which a main heater to heat the center part of a heating roller and a sub-heater to heat both ends are provided, fine timing of the on and off of these two heaters is controlled, and a reduction in life of a switching element to perform the on/off control can be suppressed.
The above respective techniques are common to each other in that plural heaters are provided in the heating roller, and the temperature control is performed by controlling the period and ratio of the on and off of each heater.
In the case where the on/off control of the plural heaters is performed, there is a possibility that the electric power consumption is significantly changed. For example, in the case where the on periods of all the heaters overlap each other, the electric power consumption becomes very large, whereas in the case where the off periods of all the heaters overlap each other, the electric power consumption approaches zero. When a variation of the electric power consumption is large, a load to a power supply circuit becomes large, and further, in order to ensure the performance also at the time when the electric power consumption is largest (in the case where the on periods of all the heaters overlap each other), a large scale power supply circuit must be provided.
Besides, although the above respective techniques are techniques to equalize the temperature of the heating roller mainly in the axial direction, in the fixing apparatus of the fixing belt system, it is important to equalize the temperature not only in the axial direction of the heating roller but also in the movement direction of the fixing belt.